To pilot an aircraft, it is necessary to know at least its relative altitude, its speed with respect to the ambient air and its angle of attack. These various data may be measured locally, in the near field, in the vicinity of the skin of the aircraft or in the far field, away from the aerodynamic field disturbed by the craft. The near-field data are obtained conventionally by measuring a certain number of aerodynamic parameters such as the static pressure allowing determination of the altitude, the total pressure allowing determination of the relative speed, the local angle of attack and the total temperature allowing the necessary corrections to be made. These measurements are performed by a certain number of probes situated on the skin of the aircraft. Mention will be made notably of static-pressure probes, pressure or “TAS” probes, the acronym standing for “True Air Speed”, temperature or “TAT” probes, the acronym standing for “Total Air Temperature”, angle of attack or “AOA” probes, the acronym standing for “Angle Of Attack”, sideslip or “SSA” probes, the acronym standing for “Side Slip Angle”, etc.
The far-field data are performed by optical anemometry devices called “LiDARs”, the acronym standing for “Light Detection And Ranging”. A LiDAR emits and receives light along a determined measurement axis. So-called “3D” conventional anemometry LiDAR architectures allowing measurements to be made in the three dimensions of space use the smallest possible number of measurement axes for obvious cost reasons. For fixed measurement axis systems, three measurement axes suffice if performing so-called “long range” measurements, that is to say without any appreciable effect of the local aerodynamic field. Otherwise, four measurement axes are necessary if the measurements are performed at short range where the boundary layer effects mean that the local flux is no longer representative of all the variations of the parameters of “the upstream infinity”.
FIG. 1 represents the diagram of a system for anemobarometric measurements comprising at one and the same time near-field measurement sensors and “LiDARs”. For obvious safety reasons, the craft has three autonomous measurement channels.
Channels 1 and 3, to the left and at the centre of FIG. 1, essentially comprise the following pneumatic probes and sensors: a Pitot 11 and 31, an angle of attack probe 12 and 32, a sideslip probe 13 and 33, a temperature probe 14 and 24 (dual-element probe shared on two channels) and two static-pressure probes 15 and 16 and 35 and 36 situated on either side of the fuselage. These various sensors are represented in white and grey in FIG. 1. As seen, these channels make it possible to measure all the anemobarometric parameters necessary for piloting. These primary measurement channels are entirely conventional.
Channel 2, to the right of FIG. 1, essentially comprises the following sensors and probes: two LiDARs 21 and 22 comprising two dual-axis optical heads symbolized by black arrows, a temperature probe 24 and two static-pressure probes 25 and 26 situated on either side of the fuselage. These various sensors are represented in black in FIG. 1. The two LiDARs make it possible to ensure at one and the same time the measurements of speed and of angle of attack and of sideslip, that is to say the so-called “TAS”, “AOA” and “SSA” functions. As seen, this channel also makes it possible to measure all the anemobarometric parameters necessary for piloting.
Finally, a so-called “Standby” backup instrumentation 38 which is situated on the instrument panel of the aircraft uses an additional Pitot 31 and shares the static-pressure probes with channel 3 (static-pressure probes 35 and 36).
A conventional optical architecture 100 used to embody the LiDARs of FIG. 1 is represented in FIG. 2. In this and in the following figure, the following conventions have been adopted:                for the propagation of the light beams, the directions of propagation of the light are represented by arrows with a triangular tip,        the linear polarization of the light is represented by an arrow with a V-shaped tip when the polarization plane is in the plane of the sheet (so-called transverse electric or TE polarization) and by a centred circle when the polarization plane is perpendicular to the plane of the sheet (so-called transverse magnetic or TM polarization).        
The implementation presented in FIG. 2 corresponds to a fibred architecture, that is to say the various optical functions are linked together by means of optical fibres, and more particularly of polarization-maintaining fibres. As seen in FIG. 2, the conventional optical architecture 100 essentially comprises:                A laser source 101 providing a linearly polarized reference wave. In FIG. 2, the direction of polarization of this wave is in the plane of the sheet;        A divider device 102 splitting the polarized reference wave provided by this source 101 into two pathways: the reference pathway R in which a first luminous flux circulates and the power pathway P in which a second luminous flux circulates;        The power pathway P follows one of the two outputs of the splitter device 102. The second luminous flux of this pathway thereafter enters an amplifier or “booster” 103;        The second luminous flux thereafter passes through an optical circulator 104. The function of this circulator is to steer at one and the same time the flux of the power pathway towards the optical head 105 and the flux received towards the measurement devices 106 and 107. This circulator comprises three pathways. The first pathway corresponds to the input of the amplified luminous flux. The second pathway corresponds to the output of the said flux towards the optical head and the third pathway corresponds to the output of the backscattered luminous flux towards the measurement channel 106 and 107. Thus, the luminous flux introduced on the first pathway is transmitted with a minimum of losses towards the second pathway without being transmitted towards the third pathway whereas a luminous flux introduced by the second pathway is transmitted with a minimum of loss towards the third pathway without being transmitted towards the first pathway. A customary way of minimizing the losses of this function for a wide range of wavelengths is to use a polarization splitting device. Thus, the circulator transmits a light beam polarized linearly in the plane of the sheet of the first pathway towards the second pathway and the circulator transmits a light beam polarized linearly perpendicularly to the plane of the sheet of the second pathway towards the third pathway. By way of example, the element 104 may be a polarization splitting device associated with a quarter-wave plate contained in the projection optics 105. It should be noted that, in this type of architecture, the emission beams are polarized under circular polarization as late as possible so as to minimize the signal sources of “narcissus” type, that is to say the echoes on impurities or elements of the system, such as for example, the reflections on the various diopres encountered on the optical path of the emission and reception pathways;        On output from the circulator 104, the flux of the power pathway enters the optical head 105 which ensures a double function. On the one hand, it projects the optical flux linearly polarized in the plane of the sheet into the atmosphere at the desired measurement distance. Moreover, it gathers the flux backscattered by the particles or the aerosols contained in the measurement volume and provides as output a reception beam which enters the circulator 104 which therefore transmits towards the measurement pathway a backscattered flux linearly polarized perpendicularly to the plane of the sheet. In FIG. 2, the optical head 105 is represented conventionally by a double arrow, symbol of the lens and a plane porthole. The change of polarization may be effected through the use of a quarter-wave plate in the optical head;        A mixing device 106 of interferometric type which may be, by way of example, a polarization-maintaining coupler allows the coherent recombination of the flux emanating from the reference pathway and the return flux coming from the circulator 104;        The two beams emanating from the device 106 are directed towards the two diodes of a balanced detector 107, making it possible to circumvent the relative noise of the source, also called “RIN” for “Relative Intensity Noise”. The reference wave is oriented by the optical element 108 according to the same polarization as the signal collected or backscattered. This element 108 may be a half-wave plate. In the case where the propagation of the beams is ensured by means of optical fibres, this function may be obtained by effecting a cross weld (slow axis/fast axis) on two polarization-maintaining fibres.        
The most expensive elements of this architecture are the laser source 101, the “booster” 103, the detector 107 and the signal processing. An aircraft installation comprises four measurement axes, distributed as two dual-axis optical heads.
The installation of this type of LiDAR on aircraft suffers from several major drawbacks, detailed hereinbelow:                The angular dynamic swing of the aircraft is fairly high (generally lying between −5° to)+25°. The angular dynamic swing of the air speed vector may be doubled in local field mode, thus passing near to 60 degrees, this being very significant and having an impact on the geometric dilution. Thus, there is a risk of losing a measurement axis when it lies practically normal to the speed vector that one seeks to measure;        The measurement of the angle of attack is critical for the maintenance of the performance in terms of both precision and passband, substantially more than the measurement of the speed itself;        The necessarily disturbed near-field 4-axis measurement distributed in two places leads to a level of redundancy in the 3D measurement of almost zero, thus now precluding a genuine check of integrity of the measurement;        The placement and the orientation of the measurement axes must be such as to be sure of not inverting the sign of the speed projection to the extent that optical homodyning without frequency shift is generally used, that is to say a single source is used to generate the signal wave on the one hand and the reference wave on the other hand.        
In order to maintain the metrology performance throughout the flight domain and with a low false alarm probability, these drawbacks lead to the overdimensioning of certain characteristics of the LiDAR such as, for example, its emitted optical power so as to obtain better visibility of the signal.
The sign inversion or aliasing problem may be avoided by employing an acousto-optical modulator or a second reference laser, coherent but shifted in wavelength. These solutions are necessarily complex and expensive.